1. Field of the Invention
The present invention relates to a toner for electrophotography usable in an electrophotographic apparatus making use of an electrophotographic process such as a copier, printer, or facsimile equipment, a manufacturing method of the toner, a developer for electrophotography using the toner, and an image forming method using the developer.
2. Description of the Related Art
Regarding electrophotography methods, many methods are already known (see, for example, Japanese Patent Application Publication (JP-B) No. 42-23910, the disclosure of which is incorporated by reference herein). Generally, a fixed image is formed after plural steps of electrically forming a latent image on a surface of a photoreceptor (latent image holding member) utilizing a photoconductive substance by a variety of means, developing the formed latent image using a toner for electrophotography (hereinafter, also simply referred to as “toner”) to form a toner image, transferring the toner image on the photoreceptor surface onto a surface of a recording material such as paper via or not via an intermediate transfer body, and fixing this transferred image by heating, pressurizing, heating and pressurizing or a solvent steam. Toner remaining on the photoreceptor surface is cleaned by various methods if necessary, and is re-supplied to the aforementioned plural steps.
As a fixing technique for fixing a transferred image which has been transferred onto a surface of a recording material, a thermal roll fixing method of inserting a transfer material onto which a toner image has been transferred between a pair of rolls composed of a heating roll and a pressure roll to fix the image is common. In addition, as a similar technique, a technique in which one or both of the rolls is substituted with a belt is also known. In these techniques, an image that is fixed fast can be obtained at high speed, energy efficiency is high, and damage to the environment due to solvent volatilization or the like is minimal, as compared with other fixing methods.
On the other hand, in order to reduce the amount of energy used in a copier or printer, technology for fixing the toner with low energy is demanded. Hence, there is a strong need for toner for electrophotography capable of fixing at a low temperature.
To lower the toner fixing temperature, a technique for lowering the glass transition point of resin for toner (binder resin) is generally used. However, if the glass transition point is too low, powder aggregation (blocking) is likely to occur, and storability of toner in a fixed image surface is decreased. Thus, from a practical standpoint, the lower limit of the glass transition point is 50° C. This glass transition point is the design point of the resin for toner presently available on market, and the method of lowing the glass transition point is not enough to obtain toner capable of being fixed at lower temperature. Further, although the fixing temperature can be lowered by using a plasticizer, there is a problem since blocking occurs in the developing device or when the toner is stored.
As means for satisfying blocking prevention, image storability at a temperature of up to 60° C., and low temperature fixing performance, it has been proposed to use a crystalline resin as a binder resin in the toner, and it has been known for a long time to use a crystalline resin as toner for the purpose of blocking prevention and low temperature fixing (see, for example, Japanese Patent Application Publication (JP-B) No. 56-13943, the disclosure of which is incorporated by reference herein). Further, the technique of using crystalline resin for the purpose of offset prevention and pressure fixing has been known for a long time (see, for example, JP-B Nos. 62-39428 or 63-25335, the disclosures of which are incorporated by reference herein).
However, by using a crystalline resin alone, the strength of the resin itself is low as compared with an amorphous resin, and there is a problem with respect to powder reliability. In particular, storage at high temperature is difficult, blocking occurs in developing device, and filming is likely to occur on a photoreceptor.
To improve the strength, it is effective to mix a crystalline resin and an amorphous resin, and further it has been attempted not to dispose the crystalline resin on a surface layer by forming a capsule structure (see, for example, Japanese Patent Application Laid-Open No. 61-120161, the disclosure of which is incorporated by reference herein). However, since the covering layer is formed by attaching resin particles to an outer side, it is hard to conceal the crystalline resin completely, and if the covering rate is increased to raise the concealing rate, the crystalline resin cannot moved smoothly to the outer side, and fixing property becomes poor. In other words, it is difficult to satisfy both blocking prevention and low temperature fixing. Besides, if resins are not mixed smoothly, there is a problem in that offset is likely to occur at high temperatures.
As a technology for satisfying both low temperature fixing and blocking resistance, it is known to use a block polymer of a crystalline resin and an amorphous resin (see, for example, JP-A Nos. 62-47649 and 62-273574, the disclosures of which are incorporated by reference herein). As compared with the case of blending two resins, both low temperature fixing and blocking resistance are more likely to be achieved, but if there is no shell layer, even though the block polymer is made from a crystalline resin and an amorphous resin, external additives may be buried in portions (crystalline portions) corresponding to the crystalline resin in the block polymer, and it is difficult to stabilize the image quality for a long period and to maintain high charging property at high temperature and high humidity.
It has also been proposed to form a shell layer in mother particles in which a block polymer is used. For example, there is a method in which mother particles are coated with resin particles by a mechanical method (see, for example, JP-A Nos. 2-198457 and 4-188154, the disclosures of which are incorporated by reference herein). However, since coating layers of these toners are coated only mechanically, they are buried in a base material or shell particles are adhered only on the surface, and thus, they are likely to peel off. In particular, when it is desired to maintain favorable image quality for a long period, there is a problem in that image quality deteriorates due to peeling of shell.
Aside from such manufacturing of toners having a capsule structure by a dry process, recently, toner manufactured by a wet process, and in particular, a toner manufactured by an emulsion aggregation method including adhering latex particles to the core and forming a shell by heating and fusing in water has been proposed (see, for example, JP-A No. 2004-191927, the disclosure of which is incorporated by reference herein). The toner containing a crystalline resin and formed into a capsule structure by the emulsion aggregation method is favorable with respect to low temperature fixing property and is also excellent with respect to toner blocking and image quality maintenance. However, at the time of running at high temperature and high humidity, filming on a photoreceptor may still be observed, and excellent image quality may not be maintained for a long period in some cases. On the other hand, the present technology is not sufficient to withstand storage at high temperature and high humidity, prevent blocking in a developing device, and stably maintain image quality for a long period, while maintaining low temperature fixing property.